Protein kinase C isoenzymes in airway smooth muscle

The protein kinase C (PKC) isoenzymes expressed by bovine tracheal smooth muscle (BTSM) were identified at the protein and mRNA levels. Western immunoblot analyses reliably identified PKCα, PKCβ_I and PKCβ_II. In some experiments immunoreactive bands corresponding to PKCδ, PKCϵ and PKCθ were also labelled, whereas the γ, η and ζ isoforms of PKC were never detected. Reverse transcriptase PCR of RNA extracted from BTSM using oligonucleotide primer pairs designed to recognize unique sequences in the PKC genes for which protein was absent or not reproducibly identified by immunoblotting, amplified cDNA fragments that corresponded to the predicted sizes of PKCδ, PKCϵ and PKCζ, which was confirmed by Southern blotting. Anion-exchange chromatography of the soluble fraction of BTSM following homogenization in Ca²⁺-free buffer resolved two major peaks of activity. Using ϵ-peptide as the substrate, the first peak of activity was dependent upon Ca²⁺ and 4β-PDBu (PDBu = phorbol 12,13-dibutyrate), and represented a mixture of PKCs α, β_I and β_II. In contrast, the second peak of activity, which eluted at much higher ionic strength, also appeared to comprise a combination of conventional PKCs that were arbitrarily denoted PKCα′, PKCβ_I′ and PKCβ_II′. However, these novel enzymes were cofactor-independent and did not bind [³H]PDBu, but were equally sensitive to the PKC inhibitor GF 109203X compared with bona fide conventional PKCs, and migrated on SDS/polyacrylamide gels as 81 kDa polypeptides. Taken together, these data suggest that PKCs α′, β_I′ and β_II′ represent modified, but not proteolysed, forms of their respective native enzymes that retain antibody immunoreactivity and sensitivity to PKC inhibitors, but have lost their sensitivity to Ca²⁺ and PDBu when ϵ-peptide is used as the substrate.